Array List Insert Method

Question

I am trying to insert an element in an array. My method is supposed to increase the the size of the array by one and insert the element in the proper spot. The issue: It adds the element in the proper spot and extends the array, but it gets rid of the original element in the spot and inserts a null. My code:

 public void insert(int point, Person person){
    Person [] newList = new Person[this.size()+1];
    for(int i = 0; i < point; i++){
      newList[i] = list[i];
    }
    newList[point] = person;

    for(int i = point+1; i<this.size(); i++){
      newList[i] = list[i];

    }
   this.list = new Person[this.size()+1];
    for(int i = 0; i <this.size(); i++){
      list[i] = newList[i];
    }
  }

The array output:

> FBArrayList name = new FBArrayList()
 [DrJava Input Box]
> name.list[0] = new Person("Emma", 7)
Person@20a3d02a
> name.list[1] = new Person("Daniel", 8)
Person@6e8a93de
> name.list[2] = new Person("Bradley", 9)
Person@327556d1
> name.list[3] = new Person("Monica", 1)
Person@3d886c83
> name.list[4] = new Person("Connor", 2)
Person@76b41f9c
> name.list[5] = new Person("Fedor", 3)
Person@462a5d25
> name.insert(3, new Person("David", 4))
> for(int i = 0; i<7; i++){
System.out.println(name.list[i].getName());
}
Emma
Daniel
Bradley
David
Connor
Fedor
java.lang.NullPointerException
> name.list
{ Person@20a3d02a, Person@6e8a93de, Person@327556d1, Person@1d1a8b9, Person@76b41f9c, Person@462a5d25, null }

Any suggestions about why I am losing Monica or how I might go about fixing it.

Random thought - instead of recreating list at the end and copying over the contents of newList, why not just do list = newList? — Krease
– Krease, Commented Feb 7, 2014 at 3:48
That is a good question. I am not sure why I'm not doing it that way. Thanks @Chris — Danny2036
– Danny2036, Commented Feb 7, 2014 at 5:00

Jason · Accepted Answer · 2014-02-07 03:58:36Z

4

public void insert(int point, Person person){
  Person [] newList = new Person[this.size()+1];
  for(int i = 0; i < point; i++){
    newList[i] = list[i];
  }
  newList[point] = person;

  // this part copies the remainder of the original list to the new list after
  // your inserted entry
  for(int i = point; i < this.size(); i++){
    newList[i+1] = list[i];

  }

  this.list = newList;
}

edited Feb 7, 2014 at 3:58

answered Feb 7, 2014 at 3:51

Jason

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1 Comment

Krease Over a year ago

Thx for adding the clarification on where the fix is exactly and why it fixes it :)

Alvin Bunk · Accepted Answer · 2014-02-07 03:49:35Z

-1

You did a:

name.insert(3, new Person("David", 4))

which inserted "David" in position 3 and overwrote "Monica"!

answered Feb 7, 2014 at 3:49

Alvin Bunk

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7 Comments

Krease Over a year ago

But it's supposed to insert, not replace...how should it be fixed?

Danny2036 Over a year ago

If I am understanding you correctly: am I overwriting Monica before I move it in to the next array?

Alvin Bunk Over a year ago

Hi @Chris. Thanks for commenting, you bring up a good point. Can the author give us more information on what the FBArrayList() is and how the insert method works. Maybe this is just a standard Java ArrayList; is that correct?

Alvin Bunk Over a year ago

Hi @user3242955. That's what it looks like when you do the name.insert(3.... But I'm not sure how the FBArrayList() is implemented...

Alvin Bunk Over a year ago

Hi @user3242955. Jason's answer is the one you should use!

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